Ryanair CEO Michael O’Leary speaks during a press conference in London on August 2, 2017.

Ryanair is bracing for its biggest ever one-day strike on Friday with pilots based in five European countries set to walk out, forcing the cancellation of about one in six of its daily flights at the height of the holiday season.

Ryanair, which averted widespread strikes before Christmas by agreeing to recognize unions for the first time in its 30-year history, has been unable to quell rising protests since over slow progress in negotiating collective labor agreements.

In response to unions serving Ryanair strike notices, Europe’s largest low-cost carrier has announced in recent days the cancellations of 250 flights in and out of Germany, 104 to and from Belgium and another 42 in Sweden and its home market of Ireland, where around a quarter of its pilots were staging their fifth 24-hour walkout.

That topped the 300 flights a day it had to cancel last month when cabin crews in Belgium, Portugal and Spain escalated the staff revolt by going on strike for 48 hours.

A Dutch court also rejected a case from Ryanair seeking to block pilots in the Netherlands from joining Friday’s strike, but the Irish airline said all of its flights there would run as scheduled.

Ryanair operates more than 2,000 flights a day, serving 223 airports across 37 countries in Europe and North Africa, and insists it will not change the low-cost model that transformed the industry and has made it Europe’s most profitable airline.

It has further angered unions by threatening to move jobs away from bases affected by the stoppages, and began carrying that out in Dublin where it cut its winter fleet by 20 percent and put over 300 employees on preliminary notice.

That was because the strikes were hurting bookings, Ryanair said, and although it was too early to assess the impact elsewhere, it added that the action will hit average fares from having to move customers to flights it could otherwise have sold at a high last-minute price.